For example, a scroll-type fluid machine that is applied as a compressor of a refrigeration circuit has fixed and movable scrolls in a housing. The fixed and movable scrolls form pressure chambers in consort with each other.
The movable scroll is caused to make an orbiting movement relative to the fixed scroll. In response to this orbiting movement, a refrigerant (working fluid) within the refrigeration circuit is drawn into the pressure chamber and compressed in the pressure chamber. The compressed refrigerant is subsequently discharged from the pressure chamber through a discharge port of the compressor toward a condenser of the refrigeration circuit.
In the refrigerant compression process, the pressure of the refrigerant becomes high within the pressure chamber, so that the movable scroll is applied with high thrust load. This thrust load functions to move the movable scroll away from the fixed scroll in its axial direction.
The thrust load hampers the smooth orbiting movement of the movable scroll. The compressor is therefore provided with a thrust-receiving device, namely a thrust bearing, in between a support surface of the housing and the movable scroll. The scroll-type fluid machines disclosed in Unexamined Japanese Patent Application Publication Nos. 2005-248925, 2005-291151 and 2005-307949 each have a plurality of pressure-receiving pieces serving as a thrust bearing, the pieces being arranged in a circumferential direction. Each pressure-receiving piece is made, for example, of PPS (Poly Phenylene Sulfide) resin or the like, and is retained in a retention hole or groove that is formed in the support surface of the housing.
In each of the scroll-type fluid machines described in the above publications, the movable scroll slides against the pressure-receiving pieces. The sliding surface of the movable scroll, which slides against the pressure-receiving pieces, is generally made of an alumite film. It is known that the alumite film has a function not only as an oxidation-resistant film but also as an abrasion resistant-film for being excellent in capability of retaining lubricating oil because of its porosity.
On the other hand, the alumite film has a rough surface, so that alumite abrasion dust is produced during an initial running-in period. The abrasion dust works as abrasive, and roughens the sliding surfaces of the pressure-receiving piece and the movable scroll. Once roughened, the pressure-receiving piece is worn by abrasive wear. When the pressure-receiving piece gets thinner along with the abrasion, a gap is created between the fixed and movable scrolls. In result, the airtightness of the pressure chambers is reduced. This causes a leakage of a working fluid from the pressure chambers, leading to a degradation in compression performance. Compression efficiency and volumetric efficiency are also reduced.
One way to avoid the reduction of the compression efficiency and the volumetric efficiency is to decrease the surface roughness of the sliding surface of the movable scroll. It is bothersome, however, to grind the sliding surface of the movable scroll after the alumite film is formed. Moreover, a device for performing such a process needs to be installed, and manufacturing cost will be increased.